day 25

25/1.pl

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+use 5.20.0;
+
+my $state = 'A';
+
+use experimental qw/ smartmatch /;
+use List::AllUtils qw/ sum /;
+
+my $iteration;
+
+my @ribbon;
+my $ri = 0;
+
+my %chain = (
+    A => [ [ 1, 1, 'B' ], [ 0, -1, 'C' ] ],
+    B => [ [ 1, -1, 'A' ], [ 1, 1, 'D' ] ],
+    C => [ [ 0, -1, 'B' ], [ 0, -1, 'E' ] ],
+    D => [ [ 1, 1, 'A' ], [ 0, 1, 'B' ] ],
+    E => [ [ 1, -1, 'F' ], [ 1, -1, 'C' ] ],
+    F => [ [ 1, 1, 'D' ], [ 1, 1, 'A' ] ],
+);
+
+while( $iteration++ <= 1_2481_997 ) {
+    say $iteration unless $iteration % 100_000;
+    my $change = $chain{$state}[ $ribbon[$ri] ];
+    $ribbon[$ri] = $change->[0];
+    $ri += $change->[1];
+    if( $ri == -1 ) {
+        unshift @ribbon, 0;
+        $ri = 0;
+    }
+    $state = $change->[2];
+}
+
+say sum @ribbon;

25/input.yaml

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+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state B:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+
+In state C:
+  If the current value is 0:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state E.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state B.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state F.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.